1. When a person at a club,party or just at school moves in on someone you are trying to get with.
2. Can be used to describe a situation where a friend moved in on your girlfriend who you were trying to get back with.

1. David was trying to get with this girl at the bar but Fabio pulled a total cockblock. Now David is left with his hand.
2. Dude! Cj totally pulled a cockblock when I was trying to get back with Arri.

To cockblock an individual (usually a male) is a term used to describe the act of someone seeking to stop a person's sexual advances towards someone else. A cockblock or cockblocker is a person who engages in such obstruction or intervention.

cockblocking is a "foul act in which someone interferes with another’s attempt at finding happiness inside someone’s pants"